| Metallurgical coal, also known
as coking coal, supplies the heat and carbon used in the
steel mill process to make steel products. The high temperatures created
from the use of coke gives steel it's strength and flexibility, so it can
be used in bridges, buildings, and automobiles.
While metallurgical coal is primarily used
by steel companies, it is also used by a variety of other industrial users
to heat and power foundries, cement plants, paper mills, chemical plants
and other manufacturing and processing facilities. Metallurgical coal is
less abundant than steam coal and can be used either for making coke or as
a replacement for steam coal
Coal & Its Uses - Steel Making
Some 70% of total steel production is based on the smelting of iron ore in
blast furnaces and the subsequent refining of the iron into steel, mainly
in Basic Oxygen Furnaces (BOF). A blast furnace typically uses iron ore,
coke (made from coal), small quantities of limestone, and, where
Pulverised Coal Injection (PCI) is employed, pulverised or granulated
thermal coal.Iron ore, mined in many countries, is a mineral containing
iron oxides. Commercial ore grades usually have an iron or ferrous content
of at least 58%. Most of the ore fed to the furnace is finely ground and
then mixed and heated with coke fines to form 'sinter'. Smaller quantities
of natural lump or pelletised ores are then added.
Coke is made from coking coals, which have
certain physical properties that cause them to soften, liquefy and then
resolidify into hard but porous lumps when heated in the absence of air.
Coking coals must also have low sulphur and phosphorous contents.
Coal is carbonised in batteries of coke
ovens. The coal blend, crushed to a maximum size of 3 mm, is poured into
the top of the ovens and heated to above 1200°C over a period of 18-20
hours. The volatile contents of the coal are driven off as coke oven gas,
which is first cleaned to remove impurities and yield by-products such as
tar and benzole; then used to heat the ovens themselves and as fuel
elsewhere in the steelworks. The red-hot coke is pushed out of the ovens,
cooled and screened to remove the smaller sizes. The larger sized material
- typically above 30 mm - goes to the blast furnace, where it: supplies
carbon as a reducing agent, removing the oxygen from the ore; provides
heat to melt the iron; acts as a load-bearing but permeable layer,
supporting the burden whilst allowing the reducing gases to pass through.
Ore, coke and limestone are fed into the
top of the furnace. The hot air blast and, if PCI is installed, the
pulverised coal, are injected through nozzles into the base of the
furnace. The pulverised coal injected in this way is used as a less
expensive source of carbon and fuel. The molten iron or hot metal are
periodically tapped from the bottom of the furnace and taken directly to
the Basic Oxygen Furnace. Steel scrap and more limestone are added, and
oxygen is blown onto the liquid metal, which is 93-95% pure iron at this
stage. The reaction with the oxygen raises the temperature to 1600-1650°C
and oxidises the impurities to leave almost pure liquid steel.
Blast furnaces with PCI require 350-400 kg
of coke, made from 525-600 kg of coking coal, plus 100-200 kg of cheaper
PCI coal - around 700 kg of coal for each tonne of hot metal produced.
Furnaces without PCI use more coke, also equivalent to some 700 kg of
coal, but all of it the more expensive coking coal. As each tonne of steel
uses approximately 90% hot metal and 10% scrap, about 630 kg of coal is
used to produce 1,000 kg of steel. |
